Consider the instrumental variable regression model Yi = 0 + 1Xi + 2Wi + ui, where Xi

Question:

Consider the instrumental variable regression model
Yi = β0 + β1Xi + β2Wi + ui,
where Xi is correlated with ui and Zi is an instrument. Suppose that the first three assumptions in Key Concept 12.4 are satisfied. Which IV assumption is not satisfied when:
a. Zi is independent of (Yi, Xi, Wi)?
b. Zi = Wi?
c. Wi = l for all i?
d. Zi = Xi?